TRISTATE LOGIC TUTORIAL

In Normal logic circuits there are mainly two states of output, either LOW or HIGH. But in complex digital systems like microcomputers and microprocessors, a number of gate outputs may be required to be connected to a common line which is referred to as a bus which, in turn, may be required to drive a number of gate inputs. When a number of gate outputs are connected to the bus, some difficulties are encountered.

Totem-pole outputs cannot be connected together due to very large current drain from the supply and consequent heating of ICs which may get damaged.

Open collector outputs can be connected together with a common collector resistor connected externally. This causes the problems of loading and operation speed.

To overcome these difficulties, special circuits have been developed in which there is one more state of output, referred to as the third state or HIGH IMPEDENCE (Hi-Z) state, in addition to Low and High states. Such circuits are called Tristate or Three state Logic.

ADVANTAGES OF TRISTATE ICs

Their outputs can be connected together (paralleled) without sacrificing switching speed. This is due to the fact that a tristate output, when enabled, operates as a totem pole output with its associated low-impedence high speed characteristic.